1. Field of the Invention
The subject invention relates to television receivers, and in particular, to digital television receivers.
2. Description of the Related Art
Consumer FM receivers sometimes have a signal-strength meter which can be used to help orient an antenna for maximum received signal strength. Such an aid is never used in analog television receivers, where the signal quality has a direct relationship to the observed picture quality. In such analog television receivers, the picture quality is a continuously-varying direct measure of the signal quality.
In a digital television receiver, the quality of the displayed picture does not bear a direct, continuously-varying relationship to the received signal quality. This is due to the sharp threshold of the receiver""s error-correction capability when faced with a compromised input signal. This results in the so-called xe2x80x9ccliff effectxe2x80x9d, where the receiver performance is binary, i.e., as the received signal quality degrades, the picture is either xe2x80x9cperfectlyxe2x80x9d displayed, or there is essentially no picture at all. This creates a difficulty in orienting the receiving antenna for best performance, as there is a poor correlation between the continuously-variable antenna position and the xe2x80x9call-or-nothingxe2x80x9d displayed picture quality.
European Patent Application EP 0 818 923 discloses a digital broadcast receiving device which includes circuitry for determining a received signal quality and for displaying this received signal quality on the display screen of the receiving device. Using this displayed received signal quality, a user is able to properly position the receiving antenna to achieve a maximum received signal quality.
As is common in digital signal processing devices, e.g., compact disc players, this receiving device includes an error detecting/correcting circuit for detecting and correcting transmission errors in the broadcasting data. This circuit includes detecting means for detecting the transmission errors, calculating means for calculating the error rate from the detection results, and correcting means for correcting the detected transmission errors. This circuit calculates the number of errors that have occurred in the broadcasting data and the amount of data that has passed through the circuit. The circuit then calculates the error rate of the data, i.e., xe2x80x9cthe number of errors/the amount of passed dataxe2x80x9d and applies the error rate to a controller. The controller then converts the error rate into a received signal quality signal which is then displayed on the screen of the receiving device.
While the error rate in the received signal may be looked at as a measure of the received signal quality, Applicants have found that other factors, including multipath interference, have significant impact on the received signal quality. Multipath interference occurs when the same broadcast signal is received at different times due to different transmission paths. These different transmission paths may result from the reception of a broadcast signal directly from the transmitter, and, at the same time, the reception of the same broadcast signal having been reflected from, for example, a building or a mountain. This multipath interference causes signal distortion and degradation in system performance, due to uncorrectable errors in the digital decoder.
It is an object of the present invention to provide a signal quality indicator (SQI) signal for a digital television receiver which is sensitive to multipath interference.
This object is achieved in a device for generating a signal quality indicator (SQI) signal for indicating a signal quality of a received digital broadcast signal for a digital receiver comprising an antenna for receiving digital broadcast signals, a tuner for selectively tuning to one of said digital broadcast signals, a variable gain amplifier for amplifying the tuned signal to a predetermined level, a demodulator for demodulating the tuned signal, said demodulator having a control output for supplying an automatic gain control (AGC) signal to the variable gain amplifier, an adaptive equalizer, said adaptive equalizer generating weighting coefficients for adapting the processing in the adaptive equalizer, and signal processing circuitry for processing the equalized tuned signal, wherein the SQI signal generating device comprises means for acquiring the weighting coefficients generated in said adaptive equalizer, wherein said weighting coefficients include a main weighting coefficient and subordinate weighting coefficients; means for comparing said subordinate weighting coefficients with said main weighting coefficient; and means for generating said SQI signal for indicating the quality of said received digital broadcast signal based on said comparison.
It is known that an effective way to cope with multipath interference is to use adaptive equalization, which can restore the impaired signal by using a known training signal or the data itself. An adaptive equalizer includes a cascade arrangement of delays and a plurality of multipliers coupled to the input and output of the cascade arrangement and also to the nodes between the delays. The outputs from these multipliers are then combined in a summing arrangement, the output from the summing arrangement forming the output of the adaptive equalizer. Depending on the amount of multipath interference, the adaptive equalizer generates weighting coefficients for the multipliers.
Applicants have found that the amount of work being performed by the adaptive equalizer is a measure of the amount of multipath interference, and as such, an indication of the quality of the received digital broadcast. To that end, a comparison of the main weighting coefficient to the other subordinate weighting coefficients is thus a measure of the amount of work being performed by the adaptive equalizer.
In a modified embodiment of the invention, the device is characterized in that said comparing means comprises means for determining an RMS value for each of the weighting coefficients; means for forming a sum of the RMS values for the subordinate weighting coefficients; and means for dividing the RMS value of the main weighting coefficient by the sum of the RMS values for the subordinate weighting coefficients thereby forming the SQI signal, whereby the higher the value of the SQI signal, the less the adaptive equalizer is working and, in turn, the higher the quality of the received digital broadcast signal.
One way of comparing the weighting coefficients of the adaptive equalizer is to divide the RMS value of the main weighting coefficient by the sum of the RMS values of the subordinate weighting coefficients. As such, the higher the value of the SQI signal formed by this algorithm, the less the adaptive equalizer is working and, in turn, the higher the quality of the received digital broadcast signal.
In order to provide a visual indication of the SQI signal, the device is characterized in that the device further comprises a graphics generator for generating a quantitative signal based on said SQI signal, an output of said graphics generator being applied to a display.
Applicants have further found that while the amount that the adaptive equalizer is working is a measure of the received signal quality, a more comprehensive indication could be achieved by a combination of the working of the adaptive equalizer, the error rate, as well as other factors.
To that end, the subject invention includes a device for indicating a signal quality of a received digital broadcast video signal for a digital television receiver comprising an antenna for receiving digital broadcast video signals, a tuner for selectively tuning to one of said digital broadcast video signals, a variable gain amplifier for amplifying the tuned video signal to a predetermined level, a demodulator for demodulating the tuned video signal, said demodulator having a control output for supplying an automatic gain control (AGC) signal to the variable gain amplifier, an adaptive equalizer, said adaptive equalizer generating weighting coefficients for adapting the processing in the adaptive equalizer, an error detecting/correcting circuit for determining an error rate in the received digital broadcast video signal, video signal processing circuitry for processing the equalized tuned video signal, and a display for displaying the video signal, wherein the signal quality indicating device comprises first means for determining whether the demodulator is able to lock onto a sync sequence in the received digital broadcast video signal; second means for determining whether the adaptive equalizer is able to lock onto said sync. sequence; means for acquiring the weighting coefficients generated in said adaptive equalizer, wherein said weighting coefficients weights include a main weighting coefficient and subordinate weighting coefficients; means for comparing said subordinate weighting coefficients with said main weighting coefficient; and means for generating said SQI signal for indicating the quality of said received digital broadcast video signal based on said first determining means, said second determining means, said error rate, and said weighting coefficient comparison.